1. Field of the Invention
This invention relates to an image forming method for forming a reproduction image in a chemically and physically stable photosensitive material having spectral sensitivity in the visible region by subject it to scanning exposure with a laser beam. More particularly, it relates to an image forming method for recording an image of quality in a photosensitive material having a sharp spectral sensitivity distribution and spectral sensitivity in the visible region where a substantial sensitivity variation occurs with a wavelength variation of the laser beam, using a light source including an inexpensive semiconductor laser for emitting a steady laser beam free of a wavelength variation.
2. Prior Art
It is known to form an image in photographic silver halide photosensitive material using silver halide as a photosensitive element (simply referred to as photosensitive material, hereinafter) by scanning it with a laser beam or laser light modulated in accordance with a recording image signal, thereby achieving two-dimensional scanning exposure. This method is successful in producing a full color image at high precision in accordance with the image information given in the form of digital signals.
With the recent advance of laser diodes and semiconductor lasers (both referred to as LD, hereinafter), it becomes possible to fabricate a laser output apparatus of simple structure capable of producing a stable laser beam. There is a desire to have an image forming apparatus using such LD. In fact, various attempts have been made. For example, U.S. Pat. Nos. 4,619,892 and 4,956,702 disclose an image forming method by scanning a color photosensitive material having sensitivity in the infrared region with a laser beam emitted by LD.
Also recently developed were wavelength conversion elements using non-linear optical materials. Such elements can convert the wavelength of laser beams emitted from light sources. For example, Japanese Patent Application Kokai (JP-A) Nos. 08345/1988, 18346/1988, 19652/1988, and 19653/1988 disclose that a wavelength conversion element in the form of a so-called SHG element for generating a second harmonic is used to convert infrared light from LD into visible light, with which a conventional photosensitive material having spectral sensitivity in the visible region is scanned to form an image.
In these image forming techniques using laser beams, an image to be recorded is available as or converted into a digital image information signal and the photosensitive material is scanned with a laser beam in accordance with the image information for achieving two-dimensional scanning exposure. The image forming technique utilizing such digital image information enables synthesis of image information signals and various image processing including sharpness and correction in density and color of the original image information, affording various diversified types of image formation beyond the limit of image formation by the conventional photographic process.
The photosensitive material used in image formation as an image output medium is adapted to provide color reproduction by a subtractive color process using yellow, magenta and cyan dyes as three primary colors like conventional photographic color photosensitive material. It includes at least one layer for each of a yellow dye-forming silver halide photosensitive layer, a magenta dye-forming silver halide photosensitive layer, and a cyan dye-forming silver halide photosensitive layer formed on a support. In the above-mentioned image formation based on laser beam scanning exposure, each photosensitive layer is independently exposed with a laser beam having a wavelength corresponding to the color generating wavelength of the layer. Since generation of the three primary colors is independent for the respective colors, color generation can be freely controlled as compared with the general color photography using color negative film and color paper.
Moreover, the use of a LD as mentioned above as the laser beam light source has the advantages that the LD itself is less costly and the exposure quantity of each laser beam for image formation can be controlled by directly modulating the LD, ensuring the manufacture of a relatively inexpensive, compact apparatus capable of forming a high precision image. Many attempts have been made for producing images of quality, for example, full color images equivalent to conventional photographic images by using an LD as a light source for emitting a laser beam and subjecting a photosensitive material similar to conventional photographic photosensitive material to scanning exposure with the laser beam.
In forming an image in a photosensitive material by subjecting it to scanning exposure with a laser beam coming from an LD or similar light source, it is important to increase the sensitivity of the photosensitive material and the scanning speed of the laser beam in order to complete image formation at high sensitivity within a short time.
In general, silver halide used as a photosensitive element in photosensitive material is of the design that in order to impart sensitivity to light (laser beam) of a desired wavelength region, an organic dye molecule known as a spectral sensitizing dye is adsorbed to the silver halide to provide a spectral sensitivity distribution of a convex shape centering at the maximum photosensitive wavelength. Then to achieve image formation at a high speed, the photosensitive material or laser beam light source is preferably designed such that the oscillation wavelength of an LD used as the light source (or the wavelength of the laser beam) is incident with the spectral sensitivity maximum of the photosensitive material as closely as possible. From this aspect, we made investigations on image formation by scanning exposure with a laser beam emitting from LD, finding many problems.
When image formation by laser beam scanning exposure is carried out by using an LD having an oscillation wavelength approximate to the spectral sensitivity maximum of a photosensitive material and directly modulating the LD to control an exposure quantity, there often appear variable density streaks along the laser beam scanning direction. Such streaks become prominent particularly at gray color generated zones of intermediate density having a wide area, failing to achieve an image quality equivalent to conventional color photographs.
Where a boundary line between different density regions extends transverse to the primary scanning direction, a problem arises that if it is intended to generate color to a constant density, the density or the degree of color generation of a certain region varies depending on whether it adjoins a higher or lower density region.
Such a problem results from the temperature dependency of the output and oscillation wavelength of LD, that is, the droop phenomenon that the oscillation wavelength and output of a semiconductor laser vary during oscillation with a temperature change or the mode hopping phenomenon that the output wavelength varies discontinuously.
As previously mentioned, the photosensitive material has a convex shaped spectral sensitivity distribution. When the oscillation wavelength of LD (or the wavelength of a laser beam) is coincident with or in the vicinity of the maximum of this spectral sensitivity distribution, the photosensitive material inevitably lowers its color generation sensitivity with a shifting wavelength. If the oscillation wavelength of LD varies with a temperature change, the photosensitive material varies its sensitivity, resulting in a variation of color generation. Additionally, an output lowering of LD due to a temperature rise also causes the formed image to vary the density of color generation.
This tendency becomes prominent with general print use photosensitive material having sensitivity in the visible region as compared with photosensitive material having spectral sensitivity in the infrared region. The general print use photosensitive material having spectral sensitivity in the visible region is chemically and physically stable and easy to handle as compared with the photosensitive material having spectral sensitivity in the infrared region. As previously mentioned, a technique of converting the wavelength of a laser beam emitted from LD by means of a wavelength conversion element such as a SHG element has been established.
Accordingly, it is expected that by combining an LD and a wavelength conversion element as a light source and using a conventional photosensitive material having spectral sensitivity in the visible region as an image output medium, a color image forming system of simple construction, low cost and easy handling can be provided.
Nevertheless, because of the properties of the spectral sensitizing dye used, the general photosensitive material having spectral sensitivity in the visible region often has a spectral sensitivity peak curve which is sharper than that of the photosensitive material having spectral sensitivity in the infrared region. The above-mentioned problem, that is, the problem that the density of color generation varies with a wavelength shift due to the temperature dependency of LD becomes more prominent in this situation.
On the other hand, in forming a full color image by a laser beam in a photosensitive material using silver halide as a photosensitive element, a long development treatment time and a variation of color generation density by development treatment are considered to be limiting factors. Such limiting factors are being overcome by the recent advance of high silver chloride emulsion technology as typified by the technique disclosed in WO 87-04534. In this regard too, it is expected to apply conventional photosensitive material to the image formation technology based on digital image information using a laser beam.
However, if it is intended to form a pictorial image in a conventional photosensitive material including a high silver chloride emulsion by subjecting the photosensitive material to image exposure with a laser beam followed by rapid development treatment, the above-mentioned problem of density variation becomes more serious.